Everyone expected it was either Cap, Tony, or both that was going to be the big death. They lived.
A lot of people were not expecting Thanos to actually win. Maybe he'd come close to acquiring all the stones, and maybe one or two of the big guns die to prevent him from doing so - but that's not what happened. Hell, in my theater, and from what I gathered from different reactions..... when Thor hits him with Strombreaker - a lot assumed he was still able to stop him.
Once the snap happened - unless you were privy to the comics - you didn't know what was coming until Bucky disappeared.
So yeah, who's left? The original team. Steve, who gave up everything for Bucky, just watched him die. Tony, who became a father figure to Peter, just watched him die. Wakanda is without a king. Thor may feel responsible he didn't aim for the head, and Banner didn't turn into the Hulk to help out more. Not only that... Tony and Steve have to talk, reconcile, and team up in order to fix anything.
While we may know that not all of the *Snap* deaths are final (T'Challa, Spidey, Starlord), there's still going to be more death to come. There's going to be a price to pay with those who are left. The stakes are still there, and it's more about the journey to the conclusion. You know Spidey is coming back, but you'd be hardpressed to find someone who saw that death scene and didn't feel the emotional weight behind it.
That means the movie did its job. In the age of information being available everywhere with the internet and social media... Marvel did a hell of a job making sure they could still surprise you with the film